Beijing placed exit bans on the wife, son, and daughter of Liu Changming, a former executive of a state-owned bank in Guangzhou, when they visited their mother's ailing father in southern China this June, the Times said.

The ban stops them from leaving the country. Although the children are seemingly able to travel and act freely within China, they are effectively hostages of the state, prohibited from going home.

The wife, Sandra Han, and children — Victor and Cynthia Liu, who are 19 and 27 — are all American citizens, and entered China with their US passports.

Victor was due to be a sophomore at Georgetown University, while his sister Cynthia works at McKinsey & Company in New York.

Liu Changming is accused of being part of a $1.4 billion fraud case, and officials in Beijing are trying to get him back to face criminal charges.

The skyline of Guangzhou, where Liu Changming worked. Siu Chiu/Reuters

Han, the wife, was detained at a secret site days after their arrival in June, the Times reported.

It said the children were forbidden to leave China when they arrived at the airport at the end of their trip. Police told the children that they were neither being investigated nor charged with a crime, the Times said, but still could not go.

Cynthia Liu wrote in a letter to US national security adviser John Bolton, according to the Times: "We are being held here as a crude form of human collateral to induce someone with whom I have no contact to return to China for reasons with which I am entirely unfamiliar."

Exit bans — the practice of forbidding travelers from leaving the country — are not uncommon in China. The country's officials have imposed them on US citizens in the past to compel family members or colleagues to cooperate, the US State Department said in a travel advisory.

China is no stranger to punishing the family members of its critics. A police officer in Shanghai in 2015. Wang Zhao/AFP/Getty Images

Part of the playbook

China is no stranger to punishing the family members of critics to pressure them into complying to the country's will.

It placed the wife of Liu Xiaobo, a human rights activist, under house arrest with 24-hour surveillance for eight years, even though she hadn't done anything wrong.